1. Compounding pharmacies are legal, but are not FDA-regulated. Unlike traditional pharmacies, which dispense drugs manufactured by drug companies, compounding pharmacies create and mix their own drugs. While the ingredients they use are FDA-approved, their methods and facilities are not regulated by the FDA. In fact, until the 1950s, when the mass manufacturing of FDA-regulated drugs took off, about 80 percent of prescriptions were compounded, according to Reuters. Such pharmacies are overseen by state pharmacy boards, which may have as few as half a dozen inspectors for thousands of pharmacies.

2. Such pharmacies are more common than you might think. According to the director of the Pharmacy Compounding Accreditation Board via Reuters, there are anywhere from 3,000 to 5,000 compunding pharmacies in the United States. As many as 5 percent of prescriptions written annually are for compounded products.

3. They’re meant to make personalized doses of medicine for special needs. Two common scenarios: making child-size doses of drugs that are only available for adults, or making dye-free or gluten-free formulations for people with certain allergies; according to theNew York Times, every acute-care hospital has a compounding pharmacy on-site.

However, it seems some pharmacies, like the New England Compounding Center (NECC) implicated in the meningitis outbreak, are functioning more like mini drug manufacturers by mass-producing certain medications, especially those that have been in short supply from FDA-regulated pharmaceutical companies. The NECC’s steroid injection, for example, was shipped to 23 states and given to 14,000 patients—not exactly a single dose for a specific patient.

4. You may not always know if you’re getting a compounded drug. Regular pharmacies don’t fill prescriptions for compounded drugs; if your doctor prescribes one, you will need to get it filled at a special pharmacy, says WebMD. But if you get medicine directly from a doctor’s office or hospital, it’s not always so clear-cut, so you should ask where the drug came from. In some cases, hospital doctors may not even know whether the hospital pharmacy purchased drugs from a compounding pharmacy or a regular pharmaceutical manufacturer.

5. Be especially careful about injectable or IV drugs. Try to take FDA-approved versions of these drugs, which must be prepared under sterile conditions, when possible. If you do require a compounded prescription, make sure the pharmacy is licensed and is familiar with and has experience preparing it, recommends the Times.